1. Field of the Invention
The present invention relates to a lock-up control system for a lock-up type automatic transmission.
2. Description of the Prior Art
Automatic transmissions are provided with a torque converter in a power transmission system thereof so as to increase a torque from an engine. The torque converter has a pump impeller driven by an engine to rotate oil within the torque converter, wherein the rotation of the oil causes a turbine runner to rotate under the reaction of a stator thereby multiplying the torque (torque converter state). Therefore, the torque converter, while in use, cannot help avoiding a slip between its pump impeller and turbine runner. It results in poor power transmission efficiency that leads to a poor fuel economy. To alleviate this drawback, there has been proposed a so-called a torque converter with a lock-up means for locking the torque converter in a lock-up state, wherein during a relatively high vehicle speed operation range when a torque variation of an engine does not create a problem a turbine runner is directly connected to a pump impeller (lock-up state) thereby eliminating a slip therebetween. A lock-up type automatic transmission provided with a torque converter of this kind in a power transmission system thereof is already employed in some vehicles.
Referring to FIG. 4, there are illustrated lock-up ranges within which the lock-up means locks the torque converter in the lock-up state when the vehicle speed is higher than a predetermined vehicle speed value (lock-up vehicle speed) in each gear position. FIG. 4 shows a shift schedule diagram of a three-speed automatic transmission wherein V.sub.1, V.sub.2, and V.sub.3 are lock-up vehicle speed values for first, second and third gear positions, respectively, and V.sub.1 ', V.sub.2 ', and V.sub.3 ' are lock-up release vehicle speed values for first, second and third gear positions, respectively. In each of the gear positions the torque converter is permitted to lock up when the vehicle speed exceeds a corresponding one of the vehicle speed values V.sub.1, V.sub.2 and V.sub.3. The lock-up state of the torque converter is released when the vehicle speed decreases below the corresponding one of the lock-up release vehicle speed values V.sub.1 ', V.sub.2 ' and V.sub.3 ' so that the torque converter is allowed to operate in a converter state.
As will be understood from FIG. 4, upon gear shifting when the automotive vehicle is operating with a large load, a substantial shock will take place because the torque converter stays in the lock-up state.